Switching audio amplifiers (class-D audio amplifiers) have found increasing use in the industry in recent years, both due to the improvements in output stage switching devices and in modulation and feedback control methods. The classical switching power amplifier system includes a pulse modulator, for converting an analog or digital source into a pulse-modulated signal which is subsequently amplified by a switching power stage. A passive demodulation filter reproduces an amplified input signal from the power modulated signal. Generally, class-D amplifiers are based on variants of Pulse Width Modulation (PWM).
For high output power at the output of a class-D audio amplifier it is desired to have a high clipping level at the input of the amplifier. However, a too high clipping level on the other hand will make the class-d amplifier stop switching. If the switch frequency is completely reduced in systems containing a boot strap voltage in the driver (power stage), the boot strap capacitor must be very large if a proper voltage is to be maintained while the amplifier is not switching. Further, if the class-D amplifier includes a feedback control system the control system can saturate if the clipping level is too high and the recovery from saturation can lead to undesired modifications of the output signal, such as, e.g. distortions.
In applications where a self oscillating control system is used for modulation of the class-D audio amplifier, the switch frequency will typically drop at high output levels. In order to minimize disturbance in the audio band it is desired not to have a switch frequency within the audio band (generally 20 Hz to 20 kHz). Furthermore at low switch frequencies the output ripple voltage can increase because the demodulation filter has less attenuation at lower frequencies, and this can lead to problems with EMI (electromagnetic interference) and undesired high frequency power dissipation in the speaker. Some self oscillating systems implement error functions when the duty cycle is higher or lower than a certain level, which can induce some undesired low frequency instability oscillation.
Therefore, clipping is considered to be an important aspect when designing class-D amplifiers for high quality audio perception.
There are some class-D amplifiers known from prior art which address the clipping aspect, an example of such an amplifier can be found in e.g. U.S. Pat. No. 7,965,141 which discloses a class-D amplifier comprising a clipping control unit that clips the output PWM-signal by limiting a digital input signal.
Further, U.S. Pat. No. 6,320,465 discloses a system that measures on the supply voltage and divides by the gain in the system to get the clipping level at the input signal that will give a desired duty cycle clipping level. Because of practical limitations in the division precision, the clipping level will have some variation. A system utilizing a division by a resistor divider is in general expensive and/or complex if a good precision of the resistor matching is to be achieved. Furthermore, the divider function will measure the supply voltage of the power stage, this will often be at a high voltage and the divider circuit can therefore not be implemented in a low voltage integrated circuit. The duty cycle for a certain output voltage will change with variation in the power stage and with demodulation filter impedance; therefore the divided clipping level will give un-precise duty cycle clipping level. Moreover, variations in the amplifier gain will also give an un-precise duty cycle clipping level.
Another prior art example can be found in U.S. Pat. No. 5,506,532. The amplifier assembly disclosed therein is limiting the duty cycle directly. However, such a system introduces problems with saturation of the control system when the duty cycle at the output is limited.
There is therefore a need for an improved power conversion system addressing the clipping aspect, in particular for high definition switching audio amplifiers.